1. Field of the Invention
This invention relates to a method and system for efficiently modifying a digitally sampled voltage input waveform to produce an output signal, particularly where the output signal is used to produce a corrected audio signal.
2. Description of Prior Art
In conventional Pulse-Code Modulation (PCM) technology, a signal is periodically sampled then quantized into a discrete number of levels, such as voltage levels, and subsequently coded as a sequence of digits. For a general explanation of PCM, see Bernard Sklar, Digital Communications Fundamentals and Applications, p. 73 (1988 Prentice-Hall, Inc.). Depending upon resolution of quantization, or number of binary bits per sample, and the sampling rate selected, the PCM representation can closely approximate an original continuous or analog signal. For example, electronic compact disc players typically employ a PCM signal. The term "voltage" can relate to a sequence of PCM samples.
Conventional analog systems exist for correcting amplifier loads to overcome phase lagging characteristics of an output signal delivered to an audio output device, such as a speaker. For example, Crooks, U.S. Pat. No. 4,482,866 and Crooks, U.S. Pat. No. 4,638,258 disclose a reference load amplifier correction system that corrects for adverse analog signal characteristics, such as reactance, inertia and resonances of an analog power amplifier driven load, such as a speaker. A program voltage is applied to a reference load that resembles or simulates characteristics of a driven load. A correction voltage signal is delivered as a function of the reference load. The program voltage and the correction voltage signal are simultaneously applied to the power amplifier to simultaneously reproduce the program and correct for adverse load characteristics. The amplifier correction systems taught by U.S. Pat. Nos. 4,482,866 and 4,638,258 deliver an analog audio signal to an audio output device that unmasks high-frequency harmonics by applying a phase lead to the high-frequency signals, relative to the low-frequency and midrange-frequency signals.
Gagon, U.S. Pat. No. 5,510,752 discloses a low input signal bandwidth compressor and amplifier control circuit which has a pre-amplifier responsive to an analog program input signal, a filter circuit and an absolute value circuit. The preamplifier automatically balances the high-frequency signals and the midrange-frequency signals, resulting in a compensated signal. The filter circuit has an adjustable bandwidth which automatically adjusts the bandwidth of the filter circuit in response to a control signal. The filter circuit uses a voltage-controlled amplifier or a photo cell in combination with an integrator. The absolute value circuit emits a control signal proportional to an average peak value of an amplitude of a modified compensated signal emitted from the filter circuit. The amplifier control circuit taught by U.S. Pat. No. 5,510,752 receives at an input terminal an analog signal.
It is apparent from the teachings of the prior art that there exists a need for a method and a system for digitally processing a digital input signal into a digital output signal that can be delivered to an audio output device wherein relatively lower frequencies phase lag relatively higher frequencies. There is also a need to independently adjust a level of relatively low-frequency energy and a level of relatively high-frequency energy, particularly where the relatively high-frequency level is also controlled by the content of the digital input signal.